Split gate non-volatile flash memory cell having a silicon-metal floating gate and method of making same

ABSTRACT

A non-volatile memory cell includes a substrate of a first conductivity type with first and second spaced apart regions of a second conductivity type, forming a channel region therebetween. A select gate is insulated from and disposed over a first portion of the channel region which is adjacent to the first region. A floating gate is insulated from and disposed over a second portion of the channel region which is adjacent the second region. Metal material is formed in contact with the floating gate. A control gate is insulated from and disposed over the floating gate. An erase gate includes a first portion insulated from and disposed over the second region and is insulated from and disposed laterally adjacent to the floating gate, and a second portion insulated from and laterally adjacent to the control gate and partially extends over and vertically overlaps the floating gate.

TECHNICAL FIELD

The present invention relates to a non-volatile flash memory cell whichhas a select gate, a silicon-metal floating gate, a control gate, and anerase gate having an overhang with the floating gate.

BACKGROUND OF THE INVENTION

Split gate non-volatile flash memory cells having a select gate, afloating gate, a control gate and an erase gate are well known in theart. See for example U.S. Pat. Nos. 6,747,310 and 7,868,375. An erasegate having an overhang over the floating gate is also well know in theart. See for example, U.S. Pat. No. 5,242,848. All three of thesepatents are incorporated herein by reference in their entirety.

In order to increase performance, the floating gate can be doped withimpurities. For example, increasing the dopant level on the floatinggate can increase the erase speed of the memory cell. However, there aredrawbacks to increased doping. For example, out-diffuse of dopants froma highly doped floating gate can decrease the quality of the dielectricmaterial surrounding the floating gate. Higher dopant levels can alsocause the blunting of the floating gate tip during oxidation processes.

Accordingly, it is one of the objectives of the present invention toimprove the erase efficiency of such a memory cell without relying onhigh levels of dopant in the floating gate.

SUMMARY OF THE INVENTION

The aforementioned objectives are achieved with a non-volatile memorycell that includes a substrate of a first conductivity type, having afirst region of a second conductivity type, a second region of thesecond conductivity type spaced apart from the first region, forming achannel region therebetween, a select gate insulated from and disposedover a first portion of the channel region which is adjacent to thefirst region, a floating gate insulated from and disposed over a secondportion of the channel region which is adjacent the second region, metalmaterial formed in contact with the floating gate, a control gateinsulated from and disposed over the floating gate and an erase gatethat includes first and second portions. The first portion is insulatedfrom and disposed over the second region, and is insulated from anddisposed laterally adjacent to the floating gate. The second portion isinsulated from and laterally adjacent to the control gate, and partiallyextends over and vertically overlaps the floating gate.

A method of forming a non-volatile memory cell includes forming, in asubstrate of a first conductivity type, spaced apart first and secondregions of a second conductivity type, defining a channel regiontherebetween, forming a select gate insulated from and disposed over afirst portion of the channel region which is adjacent to the firstregion, forming a floating gate insulated from and disposed over asecond portion of the channel region which is adjacent the secondregion, forming metal material in contact with the floating gate,forming a control gate insulated from and disposed over the floatinggate, and forming an erase gate that includes first and second portions.The first portion is insulated from and disposed over the second region,and is insulated from and disposed laterally adjacent to the floatinggate. The second portion is insulated from and laterally adjacent to thecontrol gate, and partially extends over and vertically overlaps thefloating gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an improved non-volatile memory cellof the present invention.

FIGS. 2A-2C and 3A-3J are cross sectional views of a process to make oneembodiment the memory cell of the present invention.

FIG. 4 is a cross sectional view of an alternate embodiment of thememory cell of the present invention.

FIG. 5 is a cross sectional view of a second alternate embodiment of thememory cell of the present invention.

FIG. 6 is a cross sectional view of a third alternate embodiment of thememory cell of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 there is shown a cross-sectional view of an improvednon-volatile memory cell 10 of the present invention. The memory cell 10is made in a substantially single crystalline substrate 12, such assingle crystalline silicon, which is of P conductivity type. Within thesubstrate 12 is a first region 14 of a second conductivity type. If thefirst conductivity type is P then the second conductivity type is N.Spaced apart from the first region is a second region 16 of the secondconductivity type. Between the first region 14 and the second region 16is a channel region 18, which provides for the conduction of chargesbetween the first region 14 and the second region 16.

Positioned above, and spaced apart and insulated from the substrate 12is a select gate 20, also known as the word line 20. The select gate 20is positioned over a first portion of the channel region 18. The firstportion of the channel region 18, immediately abuts the first region 14.Thus, the select gate 20 has little or no overlap with the first region14. A floating gate 22 is also positioned above and is spaced apart andis insulated from the substrate 12. The floating gate 22 is positionedover a second portion of the channel region 18 and a portion of thesecond region 16. The second portion of the channel region 18 isdifferent from the first portion of the channel region 18. Thus, thefloating gate 22 is laterally spaced apart and is insulated from and isadjacent to the select gate 20. An erase gate 24 is positioned over andspaced apart from the second region 16, and is insulated from thesubstrate 12. The erase gate 24 is laterally insulated and spaced apartfrom the floating gate 22. The select gate 20 is to one side of thefloating gate 22, with the erase gate 24 to another side of the floatinggate 22. Finally, positioned above the floating gate 22 and insulatedand spaced apart therefrom is a control gate 26. The control gate 26 isinsulated and spaced apart from the erase gate 24 and the select gate20, and is positioned between the erase gate 24 and the select gate 20.Thus far, the foregoing description of the memory cell 10 is disclosedin U.S. Pat. Nos. 6,747,310 and 7,868,375.

The erase gate 24 has a portion that overhangs the floating gate 22. Theerase gate 24 comprises of two parts that are electrically connected. Inthe preferred embodiment, the two parts form a monolithic structure,although it is within the present invention that the two parts can beseparate parts and electrically connected. A first part of the erasegate 24 is laterally adjacent to the floating gate 22 and is above thesecond region 16. The first part of the erase gate 24 has an end 32 thatis closest to the floating gate 22. The second part of the erase gate 24is laterally adjacent to the control gate 26 and overhangs a portion ofthe floating gate 22 (i.e. there is partial vertical overlap of theerase gate 24 and the floating gate 22). The second part of the erasegate 24 which is laterally adjacent to the control gate 26 and overhangsthe floating gate 22 is also vertically spaced apart from the floatinggate 22.

In the improvement of the present invention, a layer of metal 36 isformed on the floating gate 22 (below and insulated from the controlgate 26). Preferably, the layer of metal 36 is formed on that portion ofthe floating gate vertically covered by the control gate, but the metallayer 36 is not formed on that portion of the floating gate verticallycovered by the erase gate 24 (i.e. in this embodiment there is novertical overlap between the erase gate 24 and the metal layer 36). Themetal layer 36 provides a much higher concentration of electrons thanhighly doped polysilicon for increased erase performance, yet withoutthe drawbacks of using highly doped polysilicon.

As described in U.S. Pat. No. 6,747,310, the memory cell 10 erases byelectrons tunneling through the Fowler-Nordheim mechanism, from thefloating gate 22 to the erase gate 24. Further, to improve the erasemechanism, the floating gate 22 may have a sharp corner 22 a closest tothe erase gate 24 (facing a notch 24 a formed therein) to enhance thelocal electrical field during erase and in turn enhance the flow ofelectrons from the corner of the floating gate 22 to the erase gate 24.By having the metal layer 36 extend across only part of the top surfaceof the floating gate (i.e. not that part of the floating gate adjacentthe erase gate 24), tunneling between the polysilicon corner of thefloating gate 22 and the polysilicon erase gate 24 is preserved.

Referring to FIGS. 2A-2C and 3A-3J there are shown cross-sectional viewsof the steps in the process to make cell 10 of the present invention.FIG. 2A shows STI isolation regions formed in the substrate, which iswell known in the art. STI insulation material 40 is deposited or formedin trenches into the substrate, whereby the insulation material 40extends above the surface of the substrate. The substrate can be P typesingle crystalline silicon. A layer of silicon dioxide 42 is formed onthe substrate 12 of P type single crystalline silicon. Thereafter afirst layer 44 of polysilicon (or amorphous silicon) is deposited orformed on the layer 42 of silicon dioxide.

A polysilicon chemical-mechanical polish (CMP) process is performed,using the tops of the STI insulation as an etch stop, to lower the topsurface of the poly layer 44, as shown in FIG. 2B. The upper surface ofthe poly layer 44 is lowered further with a poly etch. A metal materialis deposited on the structure, followed by a metal CMP etch using theSTI insulation material as an etch step. Suitable metal materialsinclude TiN, TaN, Ti, Pt, etc. The resulting structure is shown in FIG.2C.

Referring to FIG. 3A there is shown a cross sectional view orthogonal tothat of FIGS. 2A-2C (along line 3A as indicated in FIG. 2C). Anotherinsulating layer 48, such as silicon dioxide (or even a composite layer,such as ONO) is deposited or formed on the metal layer 46. A secondlayer 50 of polysilicon is then deposited or formed on the layer 48.Another layer 52 of insulator is deposited or formed on the second layer50 of polysilicon and used as a hard mask during subsequent dry etching.In the preferred embodiment, the layer 52 is a composite layer,comprising silicon nitride 52 a, silicon dioxide 52 b, and siliconnitride 52 c. The resulting structure is shown in FIG. 3A

Photoresist material 54 is deposited on the structure, and a maskingstep is formed exposing selected portions of the photoresist material.The photoresist is developed and selectively etched. The exposedportions of composite layer 52 are then anisotropically etched untilpoly layer 50 is exposed, as shown in FIG. 3B. The photoresist material54 is removed, and using the stacks of composite layer 52 as an etchmask, the second layer 50 of polysilicon, the insulating layer 48, andthe metal layer 46 are then anisotropically etched, until the poly layer44 is exposed. The resultant structure is shown in FIG. 3C. Althoughonly two “stacks” S1 and S2 are shown, it should be clear that there arenumber of such “stacks” that are separated from one another.

Silicon dioxide 56 is deposited or formed on the structure. This isfollowed by the deposition of silicon nitride layer 58. The silicondioxide 49 and silicon nitride 50 are anisotropically etched leaving aspacer 60 (which is the combination of the silicon dioxide 56 andsilicon nitride 58) around each of the stacks S1 and S2. Formation ofspacers is well known in the art, and involves the deposition of amaterial over the contour of a structure, followed by an anisotropicetch process, whereby the material is removed from horizontal surfacesof the structure, while the material remains largely intact onvertically oriented surfaces of the structure (with a rounded uppersurface). The resultant structure is shown in FIG. 3D.

A photoresist mask 62 is formed over the regions between the stacks S1and S2, and other alternating pairs stacks. For the purpose of thisdiscussion, this region between the stacks S1 and S2 will be called the“inner region” and the regions not covered by the photoresist, shall bereferred to as the “outer regions”. The exposed first polysilicon 44 inthe outer regions is anisotropically etched. The resultant structure isshown in FIG. 3E.

The photoresist material 62 is removed from the structure shown in FIG.3E. A layer of oxide is then deposited or formed over the structure,followed by an anisotropic etch leaving spacers 64 adjacent to thestacks S1 and S2, as shown in FIG. 3F. Photoresist material 66 is thendeposited and is masked leaving openings in the inner regions betweenthe stacks S1 and S2. The polysilicon 44 in the inner regions betweenthe stacks S1 and S2 (and other alternating pairs of stacks) isanisotropically etched. The resultant structure is subject to a highvoltage ion implant forming the second regions 16. The resultantstructure is shown in FIG. 3G.

The oxide spacers 64 adjacent to the stacks S1 and S2 in the innerregion are removed by e.g. a wet etch or a dry isotropic etch. This etchalso removes oxide layer 42 over the second region 16. The photoresistmaterial 66 in the outer regions of the stacks S1 and S2 is removed.Silicon dioxide 68 is deposited or formed over the structure. Thestructure is once again covered by photoresist material 70 and a maskingstep is performed exposing the outer regions of the stacks S1 and S2 andleaving photoresist material 70 covering the inner region between thestacks S1 and S2. An oxide anisotropic etch is performed, to reduce thethickness of the spacers 64 in the outer regions of the stack S1 and S2,and to completely remove any silicon dioxide from the exposed siliconsubstrate 12 in the outer regions. The resultant structure is shown inFIG. 3H.

The photoresist material 70 is removed. A thin layer 72 of silicondioxide, on the order of 20-100 angstroms, is formed on the structure.This oxide layer 72 is the gate oxide between the select gate 20 and thesubstrate 12. It also thickens oxide layer 68 in the inner region.Polysilicon is deposited over the structure, followed by an anisotropicetch to result in polysilicon spacers in the outer regions of the stackS1 and S2 which constitute the select gates 20 of two memory cells 10adjacent to one another sharing a common second region 16. In addition,the spacers within the inner regions of the stacks S1 and S2 are mergedtogether forming a single erase gate 24 which is shared by the twoadjacent memory cells 10. A layer of insulation material is deposited onthe structure, and etched anisotropically to form spacers 74 next to theselect gates 20. The resulting structure is shown in FIG. 3I.

Thereafter, an ion implant step is performed forming the first regions14. Each of these memory cells on another side share a common firstregion 14. The structure is covered by insulation material 76. Anlithographic etch process is used to create holes extending down andexposing the first regions 14. The holes are lined or filed withconductive material to form bit line contacts 78. The final structure isshown in FIG. 3J. The overhang between the erase gate 24 and floatinggate 22 enhances erase performance, as does the metal layer 36 onfloating gate 22. Specifically, the metal layer 36 provides an almostunlimited source of electrons, and thus provides sufficient carriersource for cell operation while maintaining a lower floating gate dopingto prevent outdiffusion and/or corner blunting. The inclusion of metallayer 36 also allows the floating gate (and thus the memory cell ingeneral) to be scaled down to smaller dimensions.

The operations of program, read and erase and in particular the voltagesto be applied may be the same as those as set forth in U.S. Pat. No.6,747,310, whose disclosure is incorporated herein by reference in itsentirety.

However, the operating conditions may also be different. For example,for erase operation, the following voltages may be applied.

WL (20) BL (78) SL (16) CG (26) EG (24) Select Unselect Select UnselectSelect Unselect Select Unselect Select Unselect 0 v 0 v 0 v 0 v 0 v 0 v0 v or 0 v 9-15 v 0 v −1 to or 7-9 v −10 v

During erase, a negative voltage from −1 to −10 volts may be applied tothe select control gate 26. In that event, the voltage applied to theselect erase gate 24 may be lowered down to 6-9 volts. The “overhang” ofthe erase gate 24 shields the tunneling barrier from the negativevoltage applied to the select control gate 26.

For programming, the following voltages may be applied.

WL (20) BL (78) SL (16) CG (26) EG (24) Select Unselect Select UnselectSelect Unselect Select Unselect Select Unselect 1-2 v 0 v 0.5-5uA 1.5-3v 3-6 v 0 v 6-12 v 0 v 3-9 v 0 v

During programming, the selected cell is programmed through efficienthot-electron injection with the portion of the channel under thefloating gate in inversion. The medium voltage of 3-6 volts is appliedto the select SL to generate the hot electrons. The select control gate26 and erase gate 24 are biased to a high voltage (6-9 volts) to utilizethe high coupling ratio and to maximize the voltage coupling to thefloating gate. The high voltage coupled to the floating gate induces FGchannel inversion and concentrates lateral field in the split area togenerate hot electrons more effectively. In addition, the voltagesprovide a high vertical field to attract hot electron into the floatinggate and reduce injection energy barrier.

For reading, the following voltages may be applied.

WL (20) BL (78) SL (16) CG (26) EG (24) Select Unselect Select UnselectSelect Unselect Select Unselect Select Unselect 1.5-3.7 v 0 v 0.5-1.5 v0 v 0 v 0 v 0 v-3.7 V 0 v 0 v-3.7 V 0 v

During read, depending upon the balance between program and readoperations, the voltages on the select control gate 26 and the selecterase gate 24 can be balanced because each is coupled to the floatinggate. Thus, the voltages applied to each of the select control gate 26and select erase gate 24 can be a combination of voltages ranging from 0to 3.7V to achieve optimum window. In addition, because voltage on theselect control gate is unfavorable due to the RC coupling, voltages onthe select erase gate 24 can result in a faster read operation.

FIG. 4 illustrates a first alternate embodiment. In this embodiment, themetal layer 36 extends across the entire upper surface of the floatinggate 22. Therefore, during erase, the electrons tunnel from the cornerof the metal layer 36 to the erase gate 24. In this configuration, themetal work function should not be much higher than silicon so as to notslow down the erase operation.

FIG. 5 illustrates a second alternate embodiment. In this embodiment, anadditional layer 23 of polysilicon is formed over the metal layer. Thus,the floating gate 22/23 is constituted by two layers of polysilicon22/23 with a layer of metal 36 sandwiched in-between. This configurationpreserves the poly-to-poly tunneling during erase, yet allows the metallayer 36 to extend across the full width of the floating gate.

FIG. 6 illustrates a third alternate embodiment. In this embodiment, themetal layer 36 on the top surface of the floating gate 22 is replaced bymetal spacers 80 formed against the sides of the floating gates 22. Thisconfiguration also preserves the poly-to-poly tunneling during erase.

It is to be understood that the present invention is not limited to theembodiment(s) described above and illustrated herein, but encompassesany and all variations falling within the scope of the appended claims.For example, references to the present invention herein are not intendedto limit the scope of any claim or claim term, but instead merely makereference to one or more features that may be covered by one or more ofthe claims. Materials, processes and numerical examples described aboveare exemplary only, and should not be deemed to limit the claims.Further, as is apparent from the claims and specification, not allmethod steps need be performed in the exact order illustrated orclaimed, but rather in any order that allows the proper formation of thememory cell of the present invention. Lastly, single layers of materialcould be formed as multiple layers of such or similar materials, andvice versa.

It should be noted that, as used herein, the terms “over” and “on” bothinclusively include “directly on” (no intermediate materials, elementsor space disposed therebetween) and “indirectly on” (intermediatematerials, elements or space disposed therebetween). Likewise, the term“adjacent” includes “directly adjacent” (no intermediate materials,elements or space disposed therebetween) and “indirectly adjacent”(intermediate materials, elements or space disposed there between),“mounted to” includes “directly mounted to” (no intermediate materials,elements or space disposed there between) and “indirectly mounted to”(intermediate materials, elements or spaced disposed there between), and“electrically coupled” includes “directly electrically coupled to” (nointermediate materials or elements there between that electricallyconnect the elements together) and “indirectly electrically coupled to”(intermediate materials or elements there between that electricallyconnect the elements together). For example, forming an element “over asubstrate” can include forming the element directly on the substratewith no intermediate materials/elements therebetween, as well as formingthe element indirectly on the substrate with one or more intermediatematerials/elements therebetween.

What is claimed is:
 1. A non-volatile memory cell comprising: asubstrate of a first conductivity type, having a first region of asecond conductivity type, a second region of the second conductivitytype spaced apart from the first region, forming a channel regiontherebetween; a select gate insulated from and disposed over a firstportion of the channel region which is adjacent to the first region; afloating gate insulated from and disposed over a second portion of thechannel region which is adjacent the second region, wherein the floatinggate is formed of polysilicon; metal material formed in contact with thefloating gate; a control gate insulated from and disposed over thefloating gate; an erase gate that includes first and second portions,wherein: the first portion is insulated from and disposed over thesecond region, and is insulated from and disposed laterally adjacent tothe floating gate; and the second portion is insulated from andlaterally adjacent to the control gate, and partially extends over andvertically overlaps the floating gate; wherein the metal material isdisposed as a layer on a top surface of the floating gate, and whereinthe layer of the metal material extends over only a portion of the topsurface of the floating gate, wherein the layer of the metal materialdoes not extend over any portion of the top surface over which the erasegate second portion extends.
 2. A method of forming a non-volatilememory cell comprising: forming, in a substrate of a first conductivitytype, spaced apart first and second regions of a second conductivitytype, defining a channel region therebetween; forming a select gateinsulated from and disposed over a first portion of the channel regionwhich is adjacent to the first region; forming a floating gate ofpolysilicon material insulated from and disposed over a second portionof the channel region which is adjacent the second region; forming metalmaterial in contact with the floating gate; forming a control gateinsulated from and disposed over the floating gate; forming an erasegate that includes first and second portions, wherein: the first portionis insulated from and disposed over the second region, and is insulatedfrom and disposed laterally adjacent to the floating gate; and thesecond portion is insulated from and laterally adjacent to the controlgate, and partially extends over and vertically overlaps the floatinggate; wherein the metal material is disposed as a layer on a top surfaceof the floating gate, and wherein the layer of the metal materialextends over only a portion of the top surface of the floating gate,wherein the layer of the metal material does not extend over any portionof the top surface over which the erase gate second portion extends.